Don't try to pry apart the pallets and it can damage or crack the wood making it unable to use. Instead break down the pallets by using a reciprocating saw to cut through the nails to make them into planks. 

Build side panels with the full length boards using the 2x4's to connect corners staggering to more rigid.  Cut 2 of the 4 foot length boards for the ends and attach ends completing the rectangle.

Put plastic liner inside the box to keep grass and weeds from coming through and fill with potting soil. We used Barnyard Cafe compost from a local supplier that has manure & bedding from a hog barn, coffee grounds, yard trimmings & paper.  It's great for flowers, vegetables and anything else that grows. It was a lot less expensive at only $35 per cubic yard compared to buying bags of potting soil which would of cost us $80-$90. 

Plant seeds or live plants using a garden shovel and place a garden marker in front of each one so you know what they are and watch them grow!